Down-looking images of water bodies (e.g., lakes, oceans) are used to find, identify, and characterize objects below and on the water surface. Some well-known applications are: benthic mapping in shallow waters, measuring chlorophyll levels, locating navigational hazards, locating ordinance (e.g., mines), ship wrecks, marine life, maritime search and rescue, monitoring the health of coral reefs, detecting small vessels and their wakes, and tracking oil slicks. Satellite images were used to search large areas of ocean for debris related to the Mar. 8, 2014 disappearance of Malaysian Airlines flight MH370.
The reflection of sunlight and skylight from the waves obscures details and makes target viewing more difficult. Deglinting algorithms are used to subtract out the wave clutter. Kay et al. (S. Kay, J. D. Hedley, and S. Lavender, “Sun Glint of High and Low Spatial Resolution Images of Aquatic Scenes: a Review of Methods for Visible and Near-Infrared Wavelengths,” Remote Sens., 1, 697-730, 2009, doi:10.3390/rs1040697) is an excellent review of previously known deglinting algorithms.1 1 “Deglinting” is the common term used for removing sunlight reflection. The Hedley, Lynzenga, Hochberg, and Goodman algorithms (Kay et al., Sections 5.2-5.5) are the methods most closely related to the algorithms described herein.
While there are variations among the current algorithms, they all use two images: an image S, with visibility of both waves and the objects of interest, and another image R, that also has visibility of waves but little or no visibility of the objects. Subtracting the latter image from the first image removes the surface waves, or deglints:S′({right arrow over (x)})=S({right arrow over (x)})−βR({right arrow over (x)}),  (1)where the result S′({right arrow over (x)}), is the deglinted version of input image S, {right arrow over (x)} a pixel location, and β is the ratio of the ocean surface reflected radiance in S to the radiance in R.
For objects below the ocean surface, image S is obtained using a water-penetrating spectral band (usually blue-green) and R is obtained using a non-water penetrating band (red or infrared). For objects on the surface, the bands may be reversed since surface objects are often “brighter” in the infrared spectrum than in blue-green.
Eqn. 1 implicitly assumes that the two images are simultaneous in time. This is generally not the case with present day satellite imagers. Most use push broom scanners with several staggered linear array detectors, one line array for each spectral band. There is a small (0.3 to 2 s) but significant time difference between the times when each array passes over a fixed point on Earth.2 There are also time delays in “persistent surveillance mode” where consecutive images are made by repointing the imaging array to rescan an area of interest. Time delays in persistent surveillance mode are typically 10-30 s. Since waves are constantly moving, a time delay between the two images results in misalignment of the wave patterns and incomplete deglinting. 2 Some examples: French SPOT moderate resolution imaging satellite: 2 s delays between panchromatic and multispectral, US Landsat 8: 1 s delay between visible and deep infrared image. The high-resolution Worldview 2 and 3 satellites have inter-band delays of up to 0.35 s.